Diamond is not only a glamorous jewelry, but also an irreplaceable ultrahard material useful in the industry due to its highest hardness among natural materials. Diamond is the hardest substance in the nature and well known as “king of the hardness”. In addition to the ultrahardness, diamond also has excellent physical properties such as thermal sensitivity, thermal conductivity and semiconductivity, and thus has a wide range of applications including handicrafts, semiconductor appliances, geological drills, and cutting and grinding tools in the industry. However, due to anisotropy of the mechanical properties and presence of cleavage planes, there are some bottlenecks in practical applications of single crystal diamond and the scope of its applications is limited. To widen the scope of application of diamond in the industry, polycrystalline diamond and multi-crystal diamond have been synthesized.
Polycrystalline diamond can be produced by sintering natural diamond with binders. In particular, a mixture of diamond powders and binders (containing metals such as Co, Ni, and etc.) can be sintered at high temperature (e.g. 1,000-2,000° C.) and high pressure (e.g. 50,000-100,000 atm) to produce polycrystalline diamond. During the sintering, due to the addition of the binders, bridge bonds containing TiC, SiC, Fe, Co, Ni or the like as main components are formed among diamond crystals and the diamond crystals are embedded in the backbone of the bridge bonds via covalent bonds.
As compared with single crystal diamond, synthetic polycrystalline diamond has the following advantages: 1) it is isotropic and has a uniform hardness due to the disordered arrangement of crystalline grains; 2) it exhibits a higher strength, especially impact strength, allowing for a larger cutting capacity; 3) it can be made into a specific shape to adapt to different processing needs; 4) the performance of products made therefrom can be tailored to suit specific use purposes. Therefore, polycrystalline diamond is more suitable for the manufacture of cutting tools than single crystal diamond.
However, because of the presence of binders, such polycrystalline diamond has relatively low hardness (50-70 GPa) and poor thermal stability. Especially in case of that metal binders are used, when the temperature reaches 600-700° C., the diamond may be transformed into graphite under the catalytic action of metal ions, causing failure of the tool. To overcome the shortcoming brought by binders, in 2003, Japanese researchers Tetsuo Irifune et al. converted graphite directly into ultrahard multi-crystal diamond having a grain size of 10-200 nm at 12-25 GPa and 2,300-2,500° C. using ultra high pressure and high temperature (HPHT) technique (“Ultrahard polycrystalline diamond from graphite”, Nature, 421, P599-600). Such multi-crystal diamond is a colorless and transparent block with a Knoop hardness of up to 140 GPa, which is higher than that of single crystal diamond (60-100 GPa), and is not anisotropic (i.e. the properties such as hardness in each direction are the same).
In 2006, German researchers Natalia Dubrovinskaia et al. converted C60 directly into ultrahard multi-crystal diamond with a grain size of about 20 nm using HPHT at 20 GPa and 2,500 K (“Superior Wear Resistance of Aggregated Diamond Nanorods”, Nano Letters, 2006, 6, P824-826). Such multi-crystal diamond has a Knoop hardness of up to 127 GPa and fracture toughness of up to 11.1±1.2 MPa·m0.5 which is 2-3 higher than that of single crystal diamond (3.4-5.0 MPa·m0.5). Thus, size reduction of the crystalline grains seems to be an effective way to improve the performance of multi-crystal diamond.
Another type of synthetic diamond is diamond obtained using onion carbon as raw materials. Most of the onion carbon materials are obtained by high-temperature treatment of diamond nanopowders. For example, U.S. patent application Ser. No. 09/818,594 published on Jul. 29, 2003 under Publication No. U.S. Pat. No. 6,599,492B2 and V L Kuznetsov et al., “Onion-like carbon from ultra-disperse diamond”, Chemical Physics Letters, 1994, 222, P343-348 describe in details the methods for preparation of onion carbon. There is usually a diamond core at the center of the onion carbon obtained by such methods. Onion carbon can be transformed into diamond under HPHT conditions. In 2009, Mingzhi Wang et al. transformed such onion carbon into polycrystalline diamond at 2-6 GPa and 1,000-1,600° C. (Chinese Patent Application No. 200910175257.X published on Jun. 9, 2010 under Publication No. CN101723358A). The onion carbon as employed has diamond core and the resulting polycrystalline diamond bulk has a grain size of less than 20 nm and Vickers hardness of 45-61 GPa, which is relatively low.